Aircraft accessories (generator(s), fuel pump(s), oil pump(s), etc.) can be driven by an auxiliary power unit (APU) so as to allow driving of the accessories independently of the rotatable load (helicopter rotor, propeller, etc.) driven by the main aircraft engine(s). A starter/generator is also typically required to start the main engine(s) because of the high inertia of the rotatable load. In some configurations, the accessories are in selective driving engagement with an engine and with the engine starter/generator, so that the accessories can be driven at and/or before engine start-up.